The invention relates to a process and an apparatus for the continuous crystallization of plastic granules, particularly, amorphous plastic granules.
It is known that with homopolymers or copolymers based on polyethylene terephthalate (PET) on passing from the amorphous into the crystalline state in the temperature range approximately 70.degree. to 160.degree. C., there is a softening and a partial melting, so that the granules have a varying tendency to strongly stick together. On passing through this sticking phase it is necessary to keep the individual granules permanently moving and to avoid longer contact times, which would lead to the fusing together to form larger agglomerates. In order to attain and maintain such an intense movement the prior art provides a number of different processes.
In the conventional fluidized bed a hot gas flows through the granule bed in a quantity such that the individual granules in the fluidized state are heated to the crystallization temperature and crystallized. However, this process is very energy-intensive due to the necessarily large air quantity. Experience has shown that this gentile fluidization is no longer adequate in the crystallization of modified polyester, a polyester with a higher concentration of organic or inorganic co-components, since larger agglomerates form because the stickiness is increased by these components.
An even more intense turbulence and therefore a reduction of agglomerate formation during the crystallization phase is brought about by a pulsating gas flow based on the effervescent layer process, but this process is also very energy-intensive and involves high apparatus expenditure.
A process in which the granules are kept moving by stirring or agitating members has proved satisfactory (DE 32 13 02.5). The granule bed flows through the agitated vessel from top to bottom and in a countercurrent manner a hot gas blown in to the lower part of the reactor is heated to the necessary crystallization temperature. A sticking together of the individual granules on passing through the critical sticking phase is prevented by a slowly rotating stirrer. This process has proved particularly satisfactory for homopolyesters because compared with the previously described processes it represents both an energy and an apparatus minimum. However, if according to this process a copolyester is crystallized, the mechanical movement of the granules by the mixing blades is no longer sufficient as a result of the crystallization speed reduced by the co-component and the resulting intensified and extended sticky phase, so that. agglomerate-free crystallization cannot be ensured.
The problem of the present invention is therefore to provide a process and an apparatus for the continuous crystallization of plastic granules, particularly amorphous plastic granules, in which an intense movement of the granules is ensured both in time and locally in the area where the transition from the amorphous to the partly crystalline state takes place, i.e. in that area having the greatest sticking tendency.